1. Field of the Invention
The present invention relates to an optical coupling device. In particular, the present invention relates to an optical mode converter between a first optical waveguide and a second optical waveguide in which the two waveguides have different characteristics from each other.
2. Description of the Related Art
Said optical waveguides may be fibers, for example optical fibers of transmission lines, or planar waveguides, or integrated waveguides, for example input/output waveguides of integrated optics devices such as the output waveguide of a semiconductor laser.
Preferably, said different characteristics are different dimensions of the waveguides and/or different refractive index steps between the core and the cladding of the waveguides and/or different absolute refractive indexes between the two waveguides.
An optical beam confined in an optical waveguide propagates with a predetermined mode shape, which depends on the absolute refractive index of said waveguide, on the refractive index step (or refractive index profile) between the core and the cladding of the waveguide and on the dimensions and shape of the waveguide.
The mode shape, or mode profile, defines at least one propagation mode, typically the propagation of the fundamental mode, of the optical beam into the waveguide. In two waveguides having different characteristics, the modes associated to each waveguide are different in shape and/or in dimensions. With reference to FIG. 17, to couple an optical beam propagating in the first (launching) waveguide 2, with a first mode, to a second (receiving) waveguide 3, the dimension and/or the shape of the first mode should be changed in a way to form a second mode which propagates in the second waveguide. Preferably, the coupling is realized by means of a coupling device 4 optically coupled to the first and to the second waveguide. The coupling device acts on the mode profile of first waveguide 2, for example elliptic or circular in cross section, in a way to change its dimension and/or shape in both of two orthogonal directions and make it similar to the mode profile of the second waveguide 3. With reference to the case where the waveguide receiving the beam is a planar waveguide, we shall indicate as “vertical” a direction orthogonal to the main planar surface of the waveguide device, as “horizontal” a direction parallel to the main planar surface of the waveguide device and orthogonal to the beam propagation direction. In case of symmetric coupling (e.g. non planar waveguides) “horizontal” and “vertical” stand for any two directions orthogonal to each other and to the beam propagation direction. “Over” and “under”, “top” and “bottom”, “upper” and “lower” shall have a correlative meaning.
Generally, a coupling device between two waveguides with different characteristic is desirable if the coupling loss between the two waveguides without use of a coupling device is more than 0.2 dB.
I. Moerman et al. in IEEE Journal of Selected Topics in Quantum Electronics, vol. 3, no. 6, December 1997, pp. 1308-1320 give a review of the fabrication technologies for the monolithic integration of tapers with III-V semiconductor devices, as well as of different taper designs and performances. According to this paper, many known mode size converters consist of overlapping waveguides with one waveguide being part of, or surrounded by, a second waveguide. The optical power is coupled from one waveguide to the other by means of a tapered transition region, in which on or both of the waveguides can be tapered. According to this paper, it is important that the taper angle in the transition region is sufficiently small, to prevent coupling of power from the fundamental mode into the higher order taper modes. In such overlapping waveguide structure it is possible to change the vertical waveguide structure just by changing the lateral dimensions along the tapered section and without changing layer thickness.
GB patent application No 2,317,023 discloses a tapered rib waveguide tapering from a large, multi-mode optical waveguide to a smaller, single-mode optical waveguide, the tapered rib waveguide comprising two portions formed of the same material.
GB patent application No 2,345,980 discloses a mode shape converter having upper and lower optical rib waveguides. The core and waveguides are made of a single medium. The tapering effect in the depth direction is obtained even though only a tapering in a width direction is made.
R. S. Fan et al, in Journal of Lightwave Technology, vol. 17, no. 3, March 1999, pp. 466-474, disclose a tapered polymer waveguide structure for coupling light between optical waveguides with differing geometries. The disclosed 3D taper structure has linear tapers and a linear index grading. There is near adiabatic propagation if the structure is built sufficiently long.
US patent application 2002/118,916 discloses a mode transformer that enables using an embedded dual taper to achieve low-loss coupling between two waveguides, one of them having a much higher index difference than the other. The mode transformer includes a first and a second dielectric channel waveguide, each including a core with a tapered region. A portion of the second tapered region is embedded within the first tapered region, and the embedded portion of the second tapered region is completely surrounded by the first tapered region in the cross-section transverse to the mode transformation direction.
R. Zengerle et al., in Journal of Vacuum Science and Technology B, vol. 9, no. 6, November/December 1991, pp. 3459-3463 describe a waveguide device for spot-size transformation of an optical beam from the 1 μm range to about 8 μm using a wedge-shaped taper based on InP. The structure has on one side a typical buried rib waveguide that is compatible with a laser waveguide. The transformation structure ends in a very thin but wide output waveguide which is butt connected to the core of the single-mode fiber.
A waveguide taper structure with a similar thin but wide output, waveguide for coupling to an optical fiber is shown by R. Zengerle et al. in Journal of Vacuum Science and Technology B, vol. 11, no. 6, November/December 1993, pp. 2641-2644. The simple two layer structure is suitable for very low-loss chip-fiber interconnections.
Applicant has remarked that the use in mode size converters of thin and wide output waveguides for coupling to optical fibers, although useful in many respects, may give rise to polarization sensitivity for the mode size converter.
Accordingly, Applicant has studied tapered optical mode converters that include an input/output coupling waveguide with a substantially square cross section. This type of coupling waveguides has, as a further advantage, a simple alignment with optical fibers. On the contrary, thin and wide coupling waveguides of the type shows, e.g., in the two papers by R. Zengerle et al. cited above, have a mode that, although substantially round in shape, shows a sharp peak in the direction orthogonal to the “thin” side of the coupling waveguide. This can make the alignment with the mode of an optical fiber critical.
Applicant has faced the problem of reducing the length of a tapered structure for a predetermined desired value of coupling efficiency, while ensuring a smooth, i.e. almost adiabatic coupling of the optical mode between the coupling waveguide and the receiving waveguide.